This invention relates to an aqueous polymerization process for forming a suspension of discrete copolymer particles of styrene and butadiene.
The development of electrostatic latent images with toner particles is well known. The level of sophistication achieved for electrostatic latent image development systems over the years has been most remarkable. For example, slow and laborious manual systems commercialized in the late 1950's have evolved into elegant high speed development systems which spew out as many as two copies per second. Consequently, the performance standards for toners during the evolution of electrostatography have become increasingly stringent. In the early manual development systems, toner and carrier particles were merely moved over an imaging surface bearing an electrostatic latent image by hand, tilting a tray containing the imaging surface and toner and carrier particles. However, in recent years, toner particles are automatically recycled many thousands of times over imaging surfaces moving at extremely high velocities. Thus durable toner materials are required to withstand the physical punishment of vigorous, prolonged and continuous use. Moreover, toner particles deposited in image configuration must now be fused in extremely short periods of time. Due to the size limitations of electrostatic copying and duplicating machines, the fusing path must be relatively short. When one attempts to increase the heat energy applied to deposited toner images for fusing purposes within the constraints of a limited fusing path to achieve adequate fixing at higher rates, one approaches the charing or kindling temperature of the substrate bearing the toner image. Attempts to shorten the fusing path by utilizing flash fusing techniques often result in the formation of noxious fumes due to decomposition of components in some toners. Further, the cost and availability of energy to operate an electrostatographic imaging system is of increasing concern to users. In addition, toner materials must possess the proper triboelectric charging properties for electrostatic latent image development and must not agglomerate during storage and transportation. Thus, there is a great need for an improved toner having stable electrical and physical properties which can endure the harsh environment of high speed electrostatographic copiers and duplicators and which can also be fused at lower energy levels.
Many attempts have been made to formulate polymers which meet the demanding requirements of modern high speed copiers and duplicators. One polymer which is being developed to exhibit properties which can meet the stringent standards of advanced copiers and duplicators is a very specific copolymer of styrene and butadiene having a limited ratio of styrene to butadiene moieties, a well defined minimum glass transition temperature (T.sub.g) and a carefully controlled weight average molecular weight (M.sub.w) range.
Copolymers of styrene and butadiene may be made by various techniques. Emulsion polymerization is the most popular polymerization process. However, emulsion polymerization processes have a number of drawbacks including complicated and difficult to control coagulation operations necessary to separate the solid polymer from the latex produced during the emulsion polymerization process. Further, such polymerization processes result in undesirable residual contaminants used to carry out the emulsion polymerization. In addition, emulsion polymerization techniques can be relatively expensive due to the complex processing steps required to form and separate the polymers.
Attempts have been made to polymerize copolymers of styrene and butadiene in an aqueous medium to form styrene-butadiene copolymer particles. Examples of these techniques are illustrated in U.S. Pat. Nos. 2,836,584, 4,169,828 and 4,170,699. Unfortunately, these processes have attendant disadvantages when used to make toner resins. For example, the process of U.S. Pat. No. 2,836,584 requires polyvinyl alcohol to prevent the formation of a latex, a redox type polymerization initiator or catalyst, and a long chain mercaptan to control the physical properties of the copolymer. Further, materials containing mercaptans emit hydrogen sulfide into the atmosphere and the sulfurous compounds are absorbed by paper substrates during flash fusing due to the decomposition of the mercaptan. The odor of hydrogen sulfide in xerographic copies renders such consumer products unacceptable. The polymerization processes described in U.S. Pat. Nos. 4,169,828 and 4,170,699 require the presence of a bisulfite or persulfate modifier in the presence of adjunct modifier such as amino acids or glutamic acid which adversely affect the electrical properties of electrostatic toners prepared with these materials.
The polymerization of at least one polymerizable ethylenic monomer suspended in an aqueous medium often requires the presence of other materials such as finely divided inorganic particles and other additives. For example, the process disclosed in U.S. Pat. No. 2,673,194 to Grim requires the presence of an anionic surface active agent and the process disclosed in U.S. Pat. No. 2,801,992 to Hutchinson et al. requires the presence of excess alkali and finely divided magnesium hydroxide.
Accordingly, there continues to be a need for an improved and more effective aqueous polymerization process for forming a suspension of distinct styrene butadiene copolymer particles which may readily be separated from the reaction medium by mere filtration. There also continues to be a need for a polymerization process which will provide toner polymers possessing properties necessary to meet the demanding requirements of modern high speed electrostatographic imaging systems.